Prefab mode vs in scene Prefab editing

Hello,
I have prefab, as example, simple cube with attached script, script has simple [Serialized] List<Materials> list,
I open my prefab from project window, and it opens the prefab's edit mode, I add materials from there to the list

If I instantiate the prefab no materials shows up, but if I drop the prefab into the scene, then add materials to the list and click override prefab, then it saves all the materials and it shows up after I instantiated it.
Why doesn't it saves all attached materials in the prefab mode ?
Auto save is checked in prefab mode
Unity 2020.3.18f1

 

anyone?